The general rule is that the thickness and length of your sideburns need to be consistent with the rest of your hair. So if your hair is very short, don’t grow your sideburns thick and long (and vice versa). I think the length should not exceed the midpoint of your ear, but that said, sideburns can look really great short or long depending on your hair and features. Just like a haircut, it’s all about maintaining balance and offsetting lines and shapes.